Project Summary The primary complaint of patients with hearing loss is the strain required to understand speech in noisy environments. This excessive `listening effort' results in stress, fatigue, and cognitive impairment. A major challenge in mitigating these effects is to identify good measures of the problem. The task-evoked pupil response is an objective and non-invasive measure of listening effort that is well established in hearing research, and increasingly used in studies of hearing loss, hearing aids, and cochlear implants. However, we have little knowledge of the physiological processes tracked by the pupil or the mechanisms of related effects on sound processing. This proposal will determine which neuromodulator(s) are released in association with task-related pupillary measures (Aim 1) and examine the impact of this pupil-indexed modulation on sound processing in auditory cortex (ACtx) of mice (Aim 2). A third aim will assess how hearing loss alters task-related pupil activity for future mechanistic study. Experiments in Aim1 will address the question: What do task-evoked pupillary responses tell us about neuromodulator release in auditory processing? The hypothesis is that ACh and NE release in ACtx are tracked by distinct components of pupillary dynamics during behavior. To test this, I will correlate pupillary measures associated with behavioral responses in a psychometric tone-in-noise detection task, to activity in cortical axons using two-photon calcium imaging. Experiments in Aim 2 will determine mechanisms of the influence of pupil-indexed neuromodulator release on auditory cortical processing. I hypothesize that NE and ACh differentially modulate aspects of spontaneous and sound-driven auditory cortical activity. To test this, I will monitor the pupil while recording membrane potentials or extracellular unit activity in ACtx. I will then optogenetically silence NE/ACh axons or locally block families of NE/ACh receptors in ACtx to dissect the influence of these modulators and their receptors on sound processing. Finally, experiments in Aim 3 will examine changes in the pupillary dynamics in mice resulting from hearing loss induced by acute noise trauma. The hypothesis is that mice with hearing loss exhibit changes in their pupil responses that are similar to those seen in humans. These altered pupillary responses after hearing loss will provide a model system for future mechanistic study of increased listening effort with hearing loss associated with pupillometric readouts. Overall, this proposal will reveal cholinergic and noradrenergic modulatory mechanisms in auditory cortex related to pupil-indexed listening effort. The results will be of interest to scientists and clinicians who: use pupillometry; study neuromodulation; are concerned with listening effort; study the processing of sounds in noisy environments; or study or treat hearing loss. In addition, this proposal will lay a technological and methodological foundation for future mechanistic study of diverse behavioral modulations of auditory processing in mouse models of normal and impaired hearing, including mechanistic dissection of increased listening effort associated with hearing loss.